The present invention relates to modification of the insert assembly of a choke valve used in connection with an underwater wellhead.
Choke valves are flow throttling devices. They function to control flow and reduce the pressure of the fluid moving through them. When the pressure is reduced the velocity of the fluid increases.
The fluid moving through a choke valve can often be severely erosive. For example, a choke valve may be used to control a gas flow containing entrained sand and moving at high pressure and velocity.
It follows that a choke valve is a critical piece of wellhead flow control equipment, which must be designed and constructed to cope with an erosive flow such as that just described.
As mentioned, choke valves are used in underwater or sub-sea service. In early times, the wellheads were located at depths less than 100 feet. It was possible for divers to manually service the choke valves. However, today wellheads may be located at depths as great as 6000 feet. At these greater depths, wellhead servicing has to be carried out using an unmanned, remotely operated vehicle, referred to as an xe2x80x9cROVxe2x80x9d.
At this point it is appropriate to shortly describe the main parts of a modern known choke valve used in sub-sea service. This valve is illustrated in FIG. 1.
The choke valve is designed to be vertically oriented in use, so that its operating parts can be removed and replaced as a unit using a vertical cable extended from surface.
The choke valve comprises the following elements:
A body is provided having a T-shaped arrangement of bores providing a horizontal side inlet, a vertical bottom outlet and a vertical chamber for containing operating components. The inlet and outlet bores have an inverted L-shaped configuration;
A generally tubular cartridge is vertically positioned in the chamber and extends across the side inlet. The cartridge side wall forms a side port connecting with the inlet;
A xe2x80x98flow trimxe2x80x99 is positioned within the bore of the cartridge.
The flow trim comprises a stationary tubular cylinder referred to as a nozzle or xe2x80x98cagexe2x80x99. The cage is seated on an internal shoulder formed by the lower end of the cartridge. It extends across the inlet and its bore is vertical, being axially aligned with the outlet. The cage has flow ports extending through its side wall. The cage flow ports communicate through the cartridge port with the inlet. Fluid enters the cage bore from the horizontal inlet through the flow ports, changes direction within the cage and leaves through the vertical body outlet. In moving through the flow ports, the fluid pressure is reduced and its velocity is increased, thereby increasing the erosiveness of the stream. The flow trim further comprises a vertically oriented, tubular, external sleeve, having one closed end. The sleeve can slide along the cage side wall to throttle the ports;
A bonnet assembly is secured to the cartridge and closes the upper end of the body chamber. The bonnet assembly is also secured to the body by clamp means which can be undone by the ROV, to release the bonnet assembly from the body;
A stem extends through an opening in the bonnet assembly and connects with the flow trim sleeve. An actuator (not shown), powered by a hydraulic system operated from surface, can rotate the stem to advance and retract the sleeve, thereby adjusting the open area of the cage flow ports.
The cartridge and its contained components, as just described, can be referred to as a xe2x80x98production insert assemblyxe2x80x99.
Now, the valve body is formed of softer, more ductile steel than is the flow trim. The reason for this is that the body needs to be machined in the course of fabrication and it also has to be able to cope with stresses. The flow trim however has harder surfaces. Typically the sleeve is formed of tungsten carbide and a tungsten carbide liner is shrink-fitted to line the cage bore. This is necessary because the flow trim is positioned at the bend of the xe2x80x9cLxe2x80x9d, where it is exposed to and temporarily contains the fluid flow when it is accelerated, is changing direction and is in a turbulent state.
When a sub-sea well is first completed, the subterranean formation containing the oil or gas will typically be at sufficient pressure to drive the produced fluid to surface. The well is referred to as a xe2x80x9cflowingxe2x80x9d well. However, over time the formation pressure will diminish. Eventually it may be desirable to inject water or other fluid into the formation, through one or more wells, to increase its pressure and maintain the formation flowing capability.
This requires that fluid be pumped under pressure or xe2x80x98injectedxe2x80x99 through the choke valve in the opposite or reverse direction. If this is done with the valve shown in FIG. 1, the fluid exits the partly closed ports of the flow trim as high velocity, angled jets that impinge against and will erode the material of the choke body.
To try to reduce the erosion when a well is converted to reverse flow, well operators have resorted to operating the choke valve with a reduced pressure drop. However this is an undesirable restriction.
The present invention is concerned with providing a reverse flow insert assembly which can replace the production insert assembly when the well is to be changed from production to injection.
In accordance with the invention, a cage, closed at one end, is positioned, in use, in a vertically oriented cartridge so that the side wall of the cage is horizontal and its open end registers and communicates with the cartridge port and the body side opening (formerly the xe2x80x98inletxe2x80x99). A flow control collar is positioned around the cage. The stem assembly is adapted to enable the rotary drive of the actuator to advance and retract the collar in a horizontal direction to throttle the cage flow ports.
As a result of this arrangement, fluid, pumped in through the bottom opening (formerly the xe2x80x98outletxe2x80x99) of the choke body, passes through the cage flow ports and is accelerated, but it is temporarily contained within the hard surface of the cage, before exiting through the cage open end as a generally linear flow. In this way, erosion of the body is reduced.
The cartridge, the horizontally arranged flow trim, the bonnet assembly and the stem assembly combine to form a reverse flow assembly. This assembly can be lowered as a unit by cable from surface, dropped into place in the choke body and be secured in place using the ROV. Thereafter fluid can be pumped in a xe2x80x98reversexe2x80x99 direction into the well, without pressure drop restrictions and with reduced erosion effect.